The present invention relates to a method for controlling a welding current in a resistance welder, and more particularly to a method for controlling a welding current of a resistance welder such as a spot welder to a target value by adjusting a firing angle of a switching element to phase-control an A.C. power supply voltage and supplying the welding current to an article to be welded through a welding transformer.
FIG. 1 schematically shows a prior art spot welder. A switching element 4 comprising anti-parallel thyristors is connected to an A.C. power supply 2 and a primary winding 8 of a welding transformer 6 is connected to the switching element 4 so that a closed circuit is formed by the A.C. power supply 2, the switching element 4 and the primary winding 8 of the welding transformer 6. A controller 10 for controlling a phase of an A.C. voltage from the A.C. power supply 2 is connected to gate terminals of the switching element 4. A pair of electrodes 14 are connected to a secondary winding 12 of the welded transformer 6. An article to be welding is held between the electrodes 14.
A voltage waveform across terminals R and S of the A.C. power supply 2 is shown by Vin in FIG. 2. By setting a firing angle of the switching element 4 to .alpha. by the controller 10, firing pulses are applied to the gates of the switching element 4 at a timing of the firing angle .alpha. so that the switching element 4 is conducted to supply a current to the transformer 6. When the current becomes zero, the switching element 4 is extinguished. As a result, a voltage waveform Vout shown in FIG. 2 appears across a terminal R.sub.2 downstream of the switching element 4 and the terminal S.sub.1 and the welding current I is supplied during a conduction angle .beta..
In such a prior art spot welder, the welding current is controlled in the following manner. FIG. 3 shows a block diagram for controlling the welding current. A rectifier 16 is connected to the A.C. power supply 2. A clamp circuit 18 having a clamp voltage setter 24 connected thereto is connected to the rectifier 16. A phase-shift pulse generator 20 having a welding current command circuit 22 connected thereto is connected to the clamp circuit 18 and the phase-shift pulse generator 20 is connected to the switching element 4.
In such a prior art system, the voltage from the A.C. power supply 2 is rectified by the rectifier 16 and supplied to the clamp circuit 18. The clamp circuit 18 clamps that portion of the voltage supplied from the rectifier 16 which is higher than a voltage preset by the clamp voltage setter 24 and supplies that portion of the voltage to the phase-shift pulse generator 20 so that a control voltage corresponding to a variation of the voltage of the A.C. power supply 2 is supplied to the phase-shift pulse generator 20. A voltage corresponding to a desired welding current is manually set to the welding current command circuit 22 connected to the phase-shift pulse generator 20. This voltage is supplied to the phase-shift pulse generator 20. The phase-shift pulse generator 20 thus receives the control voltage corresponding to the variation of the A.C. power supply 2 and the voltage preset by the welding current command circuit 22 and supplies a pulsive firing angle signal determined by those voltages to the switching element 4 so that the switching element 4 conducts in accordance with the firing angle and phase-controlled such that the target welding current is maintained irrespective of the variation of the power supply voltage.
However, a relation between the welding current and the firing angle of the phase-shift pulse generator 20 varies depending on a power-factor of a load as shown in FIG. 4. In the prior art method for controlling the welding current, the welding current is compensated only in accordance with the variation of the power supply voltage without taking the variation of the power-factor of the load into consideration. Accordingly, it provides an accurate compensation for a load of a standard power-factor but cannot provide an accurate compensation for a load of a non-standard power-factor.